Tidal forces (the difference in the pull, why high water comes twice a day, the receding Moon, and mutual locking)

What stretches is the difference in the pull

Tides do not happen because the Moon pulls. They happen because the strength of that pull differs from place to place.

The Moon pulls hardest on the near side of the Earth and least on the far side, with the centre in between.

The Earth as a whole is falling under the pull at its centre, so subtract that as the baseline. What is left points toward the Moon on the near side and away from it on the far side.

What is left is a stretch, both ways. And that is why the ocean bulges in two places: toward the Moon and directly away from it.

Which is why high water comes twice a day

Two bulges become, directly, the number of high waters.

The bulges themselves stay put, aligned with the Moon. What moves is the Earth, turning beneath them once a day.

A place on the surface passes through a bulge twice in each rotation, so high water comes twice a day.

The difference falls off as the cube of the distance. The Sun is vastly heavier than the Moon and yet, being far, raises less than half the tide. When the two line up, at new and full moon, the range is greatest.

The bulge does not sit under the Moon

So far the bulge has been placed directly under the Moon. In fact it is displaced.

The Earth spins far faster than the Moon goes round. The ocean is dragged along by that spin, so the bulge is carried a little ahead of the Moon.

It is displaced because sea grinds against sea floor. Without that friction the bulge would return under the Moon instantly. Friction is what carries it past before it can settle.

The lead is about degrees. Everything in the next scene follows from that small angle.

The Moon moves away and the Earth slows

The bulge is ahead of the Moon. A bulge is mass, so it pulls the Moon forward. Pulled forward, the Moon gains energy and moves to a wider orbit.

The reaction to that same force drags the bulge of the Earth backward, so the spin of the Earth slows. The rotation the Earth loses is passing into the orbit of the Moon.

The Moon is receding by cm a year, measured by bouncing lasers off the reflectors the Apollo missions left on its surface.

The day lengthens by about milliseconds a century. Coral fossils from million years ago carry some daily bands to a year, putting the day then at about hours.

Where it ends

The spin slows and the orbit widens. Neither continues forever.

When the spin of the Earth has slowed to match the orbit of the Moon, the tides stop. The bulge no longer moves, so there is nothing left to grind. The pair settles with each showing the other one face.

The Moon is already there. It turns once per lap and keeps the same face toward the Earth. The lighter body stops first, which is only to be expected.

Both reaching that state would take some billion years, and the Sun will have swollen long before. Pluto and Charon have arrived: they are the only pair known to face each other permanently.